Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Essentially all flexographic newspaper producers today use high-pressure water spray to process their printing plates. Examples of these systems are described in U.S. Pat. No. 4,196,018 to Inoko et al. and in U.S. Pat. No. 4,801,815 to Horner. While this approach is fast, it has a number of deficiencies. First, this approach requires plumbing to transport the water to the process and energy to heat the water used in the process. Also, the process generates aqueous effluent, which must then be disposed. Often, treatment is needed before the effluent is disposed of, adding to the expense. Finally, the special polymer chemistry needed to obtain water dispersibility may limit the durability and resolution of the photopolymer resin of the printing plate. Thus, it is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images, in order to go from plate to press more quickly and to avoid the deficiencies of the solvent-based processes of the prior art.
Thermal blotting is a process that has been gaining popularity for the production of flexographic plates. It is a user-friendly process that produces a high quality plate. However, in its current implementation, it is very slow—too slow for processing newspapers and other publications. In the thermal blotting process, photopolymer printing plates are prepared using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. No. 5,279,697 to Peterson et al., U.S. Pat. No. 5,175,072 to Martens, U.S. patent publication No. U.S. 2003/0180655 to Fan et al., and in U.S. patent publication No. 2003/0211423 to Mengel et al. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. However, additional modifications are needed to the speed and efficiency of the process to allow for its use of the process in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
The thermal plate processors currently in use for packaging flexographic plates are not appropriate for flexographic newspaper plates. The main problem is that they are too slow, by at least about an order of magnitude because only one blotting station is used. In the current process, the flexographic plate is affixed to a drum or belt and passed multiple times through the blotter station until the uncured resin is removed. Preferably, the radiation curable layer is contacted with a preheated web of absorbent material. The heat in the absorbent web is transferred to the radiation curable layer upon contact, and the temperature of the radiation curable layer is raised to a temperature sufficient to enable the uncured portions of the radiation curable layer to soften or liquefy and be absorbed into the absorbent web. While still in the heated condition, the absorbent sheet material is separated from the cured radiation curable layer in contact with the support layer to reveal the relief structure. The printing plate is then manually removed from the apparatus and placed into a separate machine for post curing.
U.S. Pat. No. 5,279,697 to Peterson et al. depicts a typical thermal development device using a hot roller and an absorbent material that is contacted with the heated printing plate. This device requires that the printing element be passed multiple times through the hot rollers to remove all of the molten polymer from the surface of the printing element. U.S. Pat. No. 6,551,759 to Daems et al. suggests the use of a laminator for thermal development, whereby the printing plate is contacted with a cleaning cloth and passed through lamination rollers. Again, multiple passes of the printing plate through the device are required to remove all of the uncured photopolymer.
The present invention provides a novel system for the rapid thermal development of flexographic newspaper plates that overcomes many of the disadvantages of the prior art.
The present invention is directed to a novel system for thermally developing flexographic printing plates comprising a plurality of consecutively arranged blotting stations, each of said blotting stations comprising a pair of rolls forming a nip gap between which the imaged and exposed flexographic printing plate is passed along with means for heating at least one of the rolls in each of the plurality of blotting stations and means for conveying the flexographic printing plate through each of the plurality of consecutively arranged blotting stations. Unlike thermal development systems of the prior art, the system of the present invention achieves complete removal of resin from the printing element in only one pass.
The thermal development system of the present invention also provides numerous advantages over high pressure water spray developing systems of the prior art.
The novel system of the invention does not use water. Thus, no water hook-ups are required and there is no aqueous waste that needs to be treated and disposed. The lack of solvent (i.e., water) also means that no drying step is required, shortening the plate making process. Finally, with no requirement for water dispersibility, the chemical formulation of the printing plates is greatly expanded. A wider range of monomers, polymers, and additive can be used to achieve the desired performance characteristics of the flexographic printing plate, resulting in higher performance (e.g., on-press lifetime and resolution) and a lower resin cost.
The system of the instant invention is very fast and is set up as a single-pass design, intended for high volume users. Productivities of greater than 150 ft2/hour are possible, and single plates can be ready in as little as five minutes. Each blotting station of the present invention can be individually optimized for temperature, nip gap, and compliant roll durometer/thickness, enabling greater flexibility in processing. Finally, post-curing is built into the process. After bending and punching, if needed, the printing plate is ready for press.